Damaged areas may also absorb moisture, which will prevent achieving a successful composite repair. Solid laminates contaminated with fuel, oil, etc. may be treated by wiping THOROUGHLY with a solvent, using reagent grade solvents for the final wipe. It is important to know what the composite is made of, what the contaminant is, and to check any and all suggested procedures to ensure the solvent being used is capable of dissolving the foreign fluid without further damaging the composite structure.
If the core in a sandwich structure is contaminated, replacement of the affected material is the best answer. Solvent cleaning may be possible via a vapour degreasing process.
All affected composite materials must be dried before an effective repair can be achieved. Cured resin as well as fibers will absorb moisture from the environment, and honeycomb cores can hold large quantities of fluid. If performing a repair using high-temperature curing resin or prepreg, all moisture must be removed to prevent steam from forming and disbanding the repair.
When repairing a structure with honeycomb core, the core is almost always moisture contaminated. Refer to the drying cycles specified in repair manuals and guidelines, but note that one hour is usually not enough and sometimes 4 days of drying time is required. Drying is typically done with a heat blanket and vacuum bag, using heat to convert the moisture to steam and vacuum to draw it out. One trick for telling when the structure is dry is to exhaust the vacuum pump through a desiccant which will change color with change in moisture content.
Published courtesy of Abaris Training Resources, Inc